XUE. Modeling MIR Molecular Gas Tracers of Truncation in Highly Irradiated Planet-forming Disks

Abstract High-mass star-forming regions (SFRs), dominated by intense far-ultraviolet (FUV) radiation fields (∼10 5 G 0 ) from nearby O stars, are the birthplaces of most planetary systems. While our understanding of protoplanetary disks (PPDs) and their evolution has primarily come from observations of isolated low-mass stars in the solar neighborhood, recent JWST observations of distant high-mass SFRs have provided new insights into the inner regions of the disks surrounding young stars. The impact of strong FUV irradiation on the structure and chemistry of PPDs remains poorly constrained. Here we use the thermochemical code ProDiMo to model how disk size and polycyclic aromatic hydrocarbon (PAH) abundance influence the infrared spectra of PPDs in high-mass SFRs. Our models explore taper radii from 5 to 70 au and PAH abundances from 10 −4 to 10 −2 relative to the interstellar medium, under both strong external irradiation (10 5 G 0 ) and isolated conditions ( G 0 ). We find that line-to-continuum ratios and integrated fluxes of key molecules, such as <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="normal">HCN</mml:mi> </mml:math> and C 2 H 2 , decrease by up to 90% in the most compact, irradiated disks, indicating a strong dependence on disk truncation. PAH abundance primarily affects the continuum and PAH feature strengths in highly irradiated models. These results provide a framework for interpreting JWST spectra of PPDs in high-mass SFRs and underscore the critical roles of disk size and external FUV irradiation in shaping disk chemistry and evolution.